There is a variability in weight loss after gastric bypass surgery for obesity despite a standardised surgical procedure. This study aims to investigate if genetic markers can predict outcome after gastric by-pass surgery in treatment of obesity. The investigators will also study if adenovirus infection can influence the outcome after surgery. In a subgroup of patients, hormonal changes after gastric by-pass surgery will be assessed. Lipid and protein patterns will be assessed in plasma and tissues. Omental and subcutaneous fat as well as liver biopsies and whole blood are stored in a biobank. The same tissues will be collected from non-obese controls undergoing gallbladder surgery.